Internal combustion engines are conventionally equipped with an aftertreatment system that includes an exhaust pipe for leading the exhaust gas from the engine to the environment, and a plurality of aftertreatment devices located in the exhaust pipe, for reducing and/or removing pollutants from the exhaust gas before discharging it in the environment.
In greater detail, a conventional aftertreatment system generally includes several aftertreatment devices, such as a Diesel Oxidation Catalyst (DOC) for oxidizing hydrocarbon (HC) and carbon monoxides (CO) into carbon dioxide (CO2) and water (H2O), and a Diesel Particulate Filter (DPF), located in the exhaust pipe downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas.
The Diesel Particulate Filter (DPF) collects liquid and solid particles in a porous substrate structure while allowing exhaust gases to flow through. As the DPF reaches its nominal storage capacity, it needs to be cleaned by a process called regeneration, during Which the exhaust gas temperature is increased substantially to create a condition whereby the soot contained in the DPF is burned (oxidized).
In order to reduce NOx emissions, aftertreatment systems may include a Lean NOx Trap (LNT) as an alternative to the DOC. A LNT is a device that is used to reduce oxides of nitrogen (NOx) and includes a catalytic converter support coated with a special wash coat containing zeolites.
Lean NOx Traps (LNT) are also subjected to periodic regeneration processes or events, whereby such regeneration processes are generally provided to release and reduce the trapped nitrogen oxides (NOx) from the LNT. Lean NOx Traps (LNT) are operated cyclically, for example by switching the engine from a lean bum operation to a rich operation, performing a regeneration event also referenced as a DeNOx regeneration.
Furthermore, internal combustion engines are currently operated with multi-injection patterns, namely for each engine cycle, a train of injection pulses is performed. A typical train of injections may start from a pilot injection pulse being followed by one or more pre-injections, by a main injection pulse, eventually terminating with one or more after and/or post injections. More specifically, fuel after-injections are fuel injections in a cylinder of the engine that occur after the Top Dead Center (TDC) of the piston.
Part of the fuel injected by means of after-injections bums inside the combustion chamber and part of it burns in an aftertreatment device, such as a DOC or as a LNT for performing the respective regenerations. After-injections therefore raise the temperature of the exhaust line and of the aftertreatment devices.
Moreover, the concept of multiple after injections in a single combustion cycle may improve the regeneration strategy of another type of aftertreatment device, namely the DPF, reducing oil dilution (due to post injections) and component stresses (due to soot cake and high thermal gradient across DOC) and allowing an effective regeneration also in the low load area (i.e. idle).
A train of multiple after injections increases the overall exhaust temperature, including the exhaust manifold of the turbine. This means that a DPF regeneration combustion operates much closer to the thermal limits of the turbine. The same effect may occur for other types of regeneration processes such as, for example, a LNT DeNOx regeneration.
Therefore the injected fuel quantity in a multi-after injection cycle is usually limited by the maximum exhaust manifold temperature that can be reached, even if such temperature is generally reached slowly.